Crystallization. The rest of the mould is self-explanatory with what has gone before, and is entirely formed by loam boards. Rapping plates have become a necessity in order to prevent injury to the pattern by the moulder. They are shewn at Fig. 734 being let into the pattern, and are screwed to receive a lifting rod as there shown. ^ Crystallization of cast iron. — During the cooling of a casting the crystals arrange themselves in lines per- pendicular to the surface, but the interior portion, being cooled more slowly, pre- serves its granular nature. Fig. 74 will shew the appearance of a bar of cast iron when broken longitudinally (the student should clearly understand that the markings are exaggerated). If the corners of the casting are made quite sharp the crystals will be abruptly turned at these places, and, meeting each other also abruptly for some distance below the surface, namely, as far down as they are formed, will create a line of fracture or portion weaker than the rest. Whether these corners be external or internal, matters not; -the same thing happens. Fig. 75 shows other examples having 're-entrant angles/ as they are called, A being a circular boss cast on a plate, and B a cylinder with flanges. It will be clear that breakage would always occur more easily at these sharp angles. When the Menai Bridge was built, the hydraulic press made • for the purpose of lifting the * tubes; had a flat bottom with pretty sharp corners, as will be understood from Fig. 76, which is a sketch of the press first used. Stephenson took the precaution of building up at each 10 inches of lift, and, had it not been for this, great damage might have occurred, for the bottom of *the press suddenly gave way, and the tube fell through a space of ten inches. Fig. 77 represents the press since adopted, the crystals being allowed to take a gradual turn, so as to leave no line of fracture. . 73d.